1. Field of the Invention
This invention relates to printing and more particularly to flexographic printing with a plate having a plurality of ink carrying cells in the solids areas and in selected halftone areas, the method for making such plate, and software for implementation of such method.
2. Description of Related Art
Flexography is a direct rotary printing method that uses resilient-relief image plates of rubber or other resilient materials including photopolymers to print an image on diverse types of materials that are typically difficult to image with traditional offset or gravure processes, such as cardboard, plastic films and virtually any type of substrate whether absorbent or non absorbent. As such it has found great applications and market potential in the packaging industry.
Flexographic printing plates are normally affixed onto a printing cylinder for printing. As shown in FIG. 1 an ink fountain pan 10 supplies ink to a metering roll 14. An optional doctor blade 12 may be used to wipe off excess ink from the metering roll to assist in controlling the amount of ink that is on the metering roll. The flexographic printing plates 16 are mounted on the printing cylinder 18. The material to be printed, usually supplied as a continuous web 19, is placed between the printing roll 18 and a backing roll 20. The flexographic printing plate is brought against the material typically with just sufficient pressure to allow contact between the relief image on the plate and the material printed.
Flexographic printing plates can be made of either vulcanized rubber or a variety of radiation sensitive polymer resins, typically sensitive to ultraviolet radiation. A well known such flexographic photosensitive polymer resin plate is Cyrel(copyright), a product of E. I. DuPont de Nemours and Co. Inc. which was introduced in the mid seventies and has since found widespread acceptance by the printing industry.
Flexography printing is a printing process whereby ink is transferred through a metering roll to the relief portions of the printing plate and therefrom in a process akin to stamping from the relief plate areas to the printed surface. In order to produce good images it is essential that the ink applied to the printed surface is applied uniformly and predictably. This in turn requires that the relief areas in the flexographic plate carry ink in a uniform layer and in predictable amounts.
The prior art has attempted to solve this problem by controlling the amount of ink applied to the printing plate using a special ink metering roll which is known as an anilox roll. Anilox rolls have on their surface a plurality of ink metering cells. These cells are small indentations arrayed in regular patterns of a predetermined frequency and of uniform depth and shape. Typically they are created by engraving the cylinder face by a mechanical process or by laser. The amount of ink delivered by the anilox roll is controlled by the screen size of the cells.
In operation ink is transferred from the ink well onto the anilox metering roll 14 filling the cells. The optional wipe blade 12 wipes off excess ink from the roll surface leaving only the cells filled. The ink from the cells is then transferred onto the flexographic plate relief areas as the anilox roll and the flexographic plate rotate in contact with one another.
Flexographic printing is what may be termed a binary system. That is, it either prints or it does not. Whenever relief areas contact the printed surface, one gets a substantially solid color area. To create a gray scale, a process called half-toning is used. This is a well known process wherein gray tones are reproduced by printing a plurality of minute solid dots per unit area and varying either the frequency of the dots per unit area or the size of the dots per unit area or both.
It has been observed, and is a well known problem in flexographic printing, that solid areas, that is areas in the image where there are no half tone dots, appear to print with less saturation and somewhat less uniformity than halftone areas representing dark image areas. Thus an area with a dot coverage of 95% to 98% appears darker than a solid area (100%). Furthermore, solid flexographic image areas tend to show a xe2x80x9chaloxe2x80x9d around the solid area, that is, a darker border around the solid image area.
As mentioned earlier, flexography""s primary application is packaging. Due to product competition, the market requirements on the printing quality of the images on the packaging are becoming very stringent. There is thus a need for flexographic printing plates that alleviate these problems and for a method preferably implemented through software, to produce such plates.
This invention alleviates the above problems through a printing plate, preferably a flexographic printing plate, comprising ink carrying cells on portions of its printing surface, said portions comprising both solid and halftone areas. The presence and size of the ink carrying cells in the halftone areas are a function of the dot sizes in the halftone areas.
Still according to this invention, the solid area comprises ink carrying cells at a first pattern and the halftone area comprises ink cells at a second pattern. The second pattern may be the same as the first pattern and the ink cell in the second pattern may be at a density per unit area that is less than the density of the ink cells produced by the first pattern in the solid areas.
In accordance with one aspect of this invention, the density of ink cell dots per unit area in the halftone areas decreases as a function of the dot size in the halftone area.
The ink carrying cells in the halftone dots are, preferably, centered in the halftone dots. Because the halftone dots are usually generated digitally in a digital environment, there is therefore also provided according to this invention, a method for generating ink carrying cells centered on digitally generated halftone dots.
It is a further object of this invention to provide a digitally imaged screened film intermediate for making the printing plate described above. The film intermediate represents an image having solid image areas and halftone image areas for use in preparing a printing plate, wherein the solid image areas reproduced on the screened film intermediate comprise a dot pattern formed by an array of a plurality of distinct dots arrayed along preselected directions said dots representing ink cells, and wherein the halftone areas also comprise ink cell dots centered on the surface of a selected number of the halftone dots. The ink cell dots in the solid areas are reproduced at a first density per unit area. The ink cell dots in the halftone areas are reproduced on the halftone dots with dimensions that are a function of the halftone dot size. The density of ink cells in the halftone area is not as high as the density of ink cells in the solids. However the ink cells in the halftone areas may be placed in an array having the same frequency and orientation as the array used in placing ink cells in the solids.
It also an object of the present invention to provide a machine readable program for use in a computer to supply an exposure device with exposure data to expose an imageable element, such program performing the steps of:
(a) reading information representing an image comprising at least one solid image area and at least one halftone image area comprising halftone dots;
(b) superposing a first ink cell pattern on said at least one solid image area;
(c) superposing a second ink cell pattern on said halftone dots of said at least one halftone area said second ink carrying cell pattern being a function of said halftone dots; and
(c) generating exposure information for said exposure device to reproduce said solid image areas with said superposed first ink cell pattern and said halftone image area with said superposed second ink cell pattern.
In somewhat more detail, this invention comprises a machine readable program for generating screened bit map image data for exposing an imageable element, such program performing the steps of:
(A) receiving digital values representing image data;
(B) identifying digital values representing solid image data
(C) screening said digital values representing solid image data using a first ink cell pattern comprising a first array of ink cells having a first size, to generate screened binary solid image data representing solid image data with superposed ink cells;
(D) identifying digital values representing halftone values less than a preselected digital halftone value;
(E) screening said digital values representing halftone values less than a preselected halftone value and generating screened normal halftone image data representing halftone dots;
(F) identifying digital values representing halftone values equal to and greater than said preselected halftone value;
(G) screening said digital values representing halftone values equal to and greater than said preselected halftone value using a second ink cell pattern comprising a second array of ink cells having a second ink cell size, and generating screened ink cell carrying halftone image data representing halftone areas wherein selected halftone dots comprise ink cells on a surface thereof and
(H) combining said:
(i) screened solid image data,
(ii) screened normal halftone image data, and
(iii) screened ink cell carrying halftone image data
to form said screened bit map image data for exposing an imageable element.